In the manufacture of sputter targets used in the semiconductor industry, and more particularly to sputter targets used in physical vapor deposition (PVD) of thin films onto complex integrated circuits and other electronic components, it is desirable to produce a sputter target that will provide film uniformity, minimal particle generation during sputtering, and desired electrical properties. Furthermore, to meet the reliability requirements for diffusion barriers or plugs of complex integrated circuits, the sputter target must have high density.
Current methods to achieve suitable sputter targets from powder materials for use in complex integrated circuits involve either hot pressing or cold isostatic pressing followed by high temperature sintering. Using either of these techniques, the density of the pressed target material is about 90% of theoretical density. By way of example, to obtain that 90% density for a tungsten-base target, the sintering process needs to proceed at a minimum temperature of 1800.degree. C. This high temperature results in a significant growth of the grains. Large grain size in sputter targets is deleterious to the uniformity of the deposited films. Thus, the sputter targets fabricated by the hot press or cold isostatic press followed by high temperature sintering have proved unreliable for use in complex integrated circuits.
Another well-known technique for achieving high density for powder products is hot isostatic pressing (HIPing). With the HIPing process, the powder packing density is normally increased from the range of 30%-50% to the range of 96%-100%. Due to significant shrinkage of the volume of the powder bed, the starting dimensions of the powder bed may not be evenly reduced by this HIPing technique. Moreover, uneven packing of the starting powder bed is very easily distorted during HIPing, especially when the ratio of radius to thickness is greater than about 3.0.
One trend in the semiconductor industry to increase production yield is to increase the size of the semiconductor wafer onto which the thin film is to be deposited. New processing is available to increase wafer sizes from 8 inches to 12 inches. This requires a proportional increase in the diameter of the sputtering targets from about 12 inches to about 20 inches, which results in an increase in the ratio of target radius to target thickness. Unfortunately, using HIPing, the greater the ratio of target radius to thickness, the easier the disk-shaped targets are distorted. To alleviate the distortion problem, one approach is to increase the powder bed density by cold isostatic pressing prior to HIPing. A second approach is to reduce the ratio of radius to thickness for the starting powder bed by increasing the thickness. The cold isostatic pressing step will increase the powder packing density from about 20-30% to about 50-60%. However, as with HIPing, cold isostatic pressing also generates significant distortion to the powder bed when the ratio of radius to thickness is greater than about 3. Therefore, to obtain the desired dimensions and shapes for the sputtering target, more powder is required by using cold plus hot isostatic pressings. The powder materials used for electronic or semiconductor applications are typically expensive due to their high purity requirements. Accordingly, the additional cold isostatic pressing process, plus the extra amount of expensive powder, significantly raises the production cost. The second approach, increasing the target thickness, also increases the cost due to the increase in powder material needed for target production.
There is thus a need to develop a method for fabricating high-density sputtering targets with large radius to thickness ratios that meet the reliability requirements for complex integrated circuits, and yet are not prohibitively expensive to manufacture.